DESCRIPTION (provided by PI): Ischemic stroke kills brain cells by causing massive depolarization and glutamate release, which in turn disrupts calcium homeostasis and generates free radicals. Mitochondria are central to these events, as well as to the wave of delayed cell death that follows the ischemic injury. Attention has focused on neuroprotective drugs that prevent the initial calcium overload, while interventions into later stages of neuronal damage have been largely neglected despite the promise of a wider treatment window. Protein phosphatase 2A (PP2A) is an essential and ubiquitous Ser/Thr phosphatase that predominately exists as a heterotrimer of two core subunits (a catalytic and a scaffolding subunit) complexed to a third, regulatory subunit Bp2 is an alternative splice variant of a PP2A regulatory subunit gene that is expressed only in the adult brain. A non-coding CAG repeat expansion in the B(3 gene causes the neurodegenerative disorder spinocerebellar ataxia type 12. The alternative N terminus of B(32 promotes translocation of PP2A to the outer mitochondrial membrane (OMM) to induce apoptosis in PC 12 cells (JBC 278:24976;JBC 280:27375), whereas interfering with endogenous mitochondrial PP2A protects neurons against ischemic insults in vitro. The proposal is centered on the observation that mitochondrial shape transitions underlie the pro- apoptotic activity of PP2A/Bp2, with overexpression of Bp2 fragmenting mitochondria and Bp2 silencing causing mitochondrial elongation in hippocampal neurons. Opposing the fission activity of the phosphatase is protein kinase A (PKA) anchored to the OMM via A-kinase anchoring protein D-AKAP1. Aim 1 test the hypothesis that OMM-localized PP2A and PKA control neuronal survival by reciprocally modulating mitochondrial fission and fusion, which in turn alters free radical and calcium homeostasis. Aim 2 explores the fission GTPase Drp1 as an effector substrate for outer-mitochondrial PP2A and PKA. In Aim 3, in vivo delivery of viruses expressing three kinds of PP2A/Bp2 antagonists is intended to provide proof-of-concept evidence that this neuron-specific pro-apoptotic protein is a promising target for stroke therapy. Finally, Aim 4 characterizes Bp2 null mice for mitochondrial changes and resistance to ischemic injury.